Method of making transducer cores



June 3, 1958 w. E. POLLOCK 2,835,381

METHOD OF MAKING TRANSDUCER CORES Filed June 3, 1953 1 2 Sheets-Sheet 1 FIG. 2.

INVENTOR s 7 9 l0 WlLLiAM E. POLLOCK y BY ATTORNEY June 3, 1958 w. E. POLLOCK 2,335,381

METHOD OF MAKING TRANSDUCER CORES Filed June 3, 1953 2 Sheets-Sheet 2 FIG. 5.

INVENTOR WiLLIAH E. POLLOCK J @aw ATTORNEY F atten-ted June 3, 1958 2,836,881 METHOD OF MAKHQG TRANSDUCER CORES William E. Pollock, Sun Valley, Califl, assignor to Librascope, Incorporated, Glendale, Calif., a corporation of California Application June 3, 1953, Serial No. 359,251

Claims. (Cl. 2--155.5)

This invention relates to the manufacture of cores for magnetic transducers, and particularly to means and a method for producing a fine gap in a sintered toroidal core.

In the electromagnetic art as applied to computing devices, the recording or reading of information is frequently accomplished by the use of a small toroidal electro-magnetic transducer head. This head consists of a toroidal core with a gap extending radially across one point in its periphery, and having windings disposed thereabout for the passage of current. The magnetic field may be distorted outwardly at the gap portion of the toroid, and it is this extending field which cooperates with the data storage medium in recording or picking up information.

The toroid itself is formed of material having high magnetic permeability, with low eddy current and hysteresis loss. It has been found that a core made by compressing material such as powdered manganese zinc ferrites at a temperature suflicient to cause sintering will have a hard, rigid structure which possesses such characteristics in a high degree.

Since the sintered ferrite is extremely hard, the slot has had to be cut in the past by means such as a diamond saw. The effectiveness of the head in recording a large number of bits of information in a small space is inversely related to the width of the gap in the toroid. The best gaps produced by diamond sawing have been of the order of one to two thousandths of an inch.

With the present invention satisfactory gaps have been produced having a width of the order of one ten-thousandths of an inch, and the gap width may be fixed at any value up to about five ten-thousandths if desired.

The fine gap is produced by the method of applying controlled stretching force in a direction tangential to the toroid at a point at which the cross-sectional area of the toroid has been previously reduced, and removing the force at the instant the fracture occurs.

It is well known that glass and ceramic articles, which have many physical characteristics in common with sintered materials, may be fractured along a desired line by scribing the surface with a diamond point, and applying a bending force transversely of the inscribed line. However, it has never before been appreciated that controlled tension might be used to separate parts of a closed figure of such rigid or semi-rigid material, such as a toroid, without fracturing it at more than one location.

A fracture at a single point may be produced, according to the present invention, with the aid of a fixture which will impart sufiicient tangential tension to break the core where the cross-section has been reduced, but which will exert no further force, once the toroid has fractured at the single desired location.

The object of the invention is thus primarily to improve the manufacture of toroidal cores of sintered materials.

Another object of the invention is to provide means and a method for producing a controlled fine gap in such cores.

A further object is the production of sintered toroidal cores with magnetic gaps of a much smaller order of magnitude than has hitherto been possible.

Yet another object is rapid and inexpensive production of sintered toroidal cores having extremely fine but accurately controlled gaps.

These and other objects may be better understood from the description following when read in conjunction with the drawings, in which:

Fig. l is a front elevational view of a fixture for fracturing a core in accordance with the method of the invention, with a dotted showing of a core in position on the fixture ready for the fracturing operation;

Fig. 2 is a side view of the fixture, taken as indicated by line 2-2 of Fig. 1;

Fig. 3 is a fragmentary sectional view taken as indicated by line 3-3 of Fig. l; and

Fig. 4 is a fragmentary sectional view taken as indicated by line 5-4 of Fig. 1.

Fig. 5 is a detailed view of a core having a gap produced by the method of the invention.

Fig. 6 is a detailed view of an alternative form of fixture for fracturing a core, shown in position to be treated.

The method here disclosed consists in reducing the cross-sectional area of the core at the desired gap location to about one-fourth its extent elsewhere by notching its internal periphery in a deep V, and scribing the exte-- rior exactly opposite the apex of the V cut, as preliminary;

steps.

The core is then clamped radially on both sides of the V, and a stretching force applied tangentially of the.

core at the notched portion.

Application of the stretching force is continued until the core fractures, and is then removed. The fracturev may later be spread to permit the insertion of a thin shim. of silver or equivalent material, but in no case should it. be permitted to a degree which will cause a second frac-- ture of the core.

A preferred form of fixture adapted to carry out the.-

method has been shown in the Figs. 1-4.

The fixture consists of a body portion 1 formed from;

reduced width body portion defines legs 6 and '7. A.

threaded bore 9 is formed in leg 7 near end 5 transversely of slot 4 and normal to the axis of bore 2. A tension. screw 10 is inserted in the threaded bore 9, in position.

to bear against leg 6.

The cross-bore 2. is of substantial size in relation to the body 1 and legs 6 and '7. Hence, when tension screw- 10 is tightened against leg 6, the legs 6 and 7 will remain; straight, but will be spread apart about the bore 2 as a center.

The stretching force is applied to the toroid by means of bosses l1 and 12 formed integrally with legs 6 and 7 respectively, and extending therefrom parallel to the axis of bore 2 far enough to provide a firm seat for the core 14 in which the gap is to be formed.

The core id is prepared by rounding off any sharp corners or extrusions remaining from the molding and sintering operation, and providing a V notch 15 in its inner periphery, having an included angle of about 69, either in the molding of the core or by cutting the same with a diamond hone. The angle selected "is one which core material.

a V r U will concentrate the magnetic field at the gap, so that the lines of force will tend to pass through the magnetic data 7 storage medium. Other angles up to about 90 have also been found satisfactory. 'The notch should be deep enough so that substantially only one-fourth of the radial extent of the toroid remains. The thickness at the bottom of the notch would be about .025".ina typical embodi' ,ment in which the toroid has an outside diameter of /2" and a toroidal cross-section approximately .093" square. The toroid i4 is then scribed on its external periphery opposite the bottom of the notch with a sharp diamond or carboloy. point, andmaybe coated with lacquer to prevent the .dislodging of small particles adjacent the scribed line during the fracturing process.

Bosses l1 and lfihave engaging faces 16 and 17 formed thereon including a corresponding 60 angle, in case. it

is desired to vary the included notch angle, a fixture must i be used having an angle betweenthe engaging faces corresponding to the newly selected notch angle. 7 I

Clamping abutments 19' and 20 are formed terminally of legs 6 and 7, respectively, extendingin the same direction from the legs 6 and 7 as do the bosses li'and ll. Threaded bores 21 and 22 are formed in clamping abut- 'ments 1? and 2% with axesintersecting at the position' which will be occupiedIby the center'of core 14. :Clampof the material of the toroid, slight as it is, will cause the.

toroid to clamp the sides of the gap tightly against the foil shim. V 1

For gaps slightly wider than can be conviently produced by such lapping, a saw of the order of .0005 .of an inch in thickness may be used in conjunction with dia- V mond dust or the like to widen the gap and freethe walls of the fracture from any incidental irregularities. Gaps narrower than the thickness of such a saw blade may be produced by first spreading the gap produced by fracture suliiciently to permit a sawed slot to be started with such a saw and only partially relaxing the force spreading the 7 gap as the sawing operation progresses. Then, when the saw has passed completely through the fractured area and has been removed therefrom, a foil shim of the order of .06025 of an inchin thickness may be inserted in the slot, and upon complete relaxation of the force tending .to spread the gap, the resiliency of the toroid material, slight as it is, will cause the toroid to clamp the sides of.

the gap tightly against the foil shim, leaving a gap, of a width of .00025 of an inch, or less than that of thersaw ing screws 24 and 25 areFdisposed in bores '21 and 22, and

arranged to engage the periphery of core 14 on opposite sides of the intended gap. Screws 24 and 25 will'hence exert their clamping effect radially of a toroidal core emplaced on the bosses 11 and 12.. V

The various elements of the fixture shown in'theifig urcs are so spaced that when a core is placed on the bosses 11 and 12, and clamping screws 24 and 25 are tightened, the tightening of tension screw 10 against leg 6 will create stress in the core in a direction substantially tangentto the center line of he core at the notched por- V,

The force will also be substantially uniform 7 tion.

throughout the reduced cross-section of the'core; Screw it? is turned until the reduced cross-section portion of core 14 hassfractured at 26, as determined byvisual observation' though. a' microscope, or equivalent means. Application'of spreading force is then stopped. p r, a

The core may be later spread to allow the insertion of-a sheet 27 of'silver foil or equivalentimaterial, which is usually of the'order of .O00l'thick'. The core will fracture opp'ositethe gap if itis spread excessively, however, and care should be takento limit the distortion to theleast amount which will. permit the ready insertion of the foil;

j In cores of the dimensions set forth above, a second frac ture Will occur'diametrically opposite the 'first,.if the gap is spread to about .302".

- When the force'tending to spread the core is removed,

the core will spring back and hold the foil 7 firmly in 7 place. The foil 27 will deform tomate with any surface 1 irregularities of the core at the fracture 26 and the gap will then be of known length. Any excessfoil protruding beyond the core should be trimmed away By further working of the fracture gap produced in the manner describedherein, it 'is possible'to produce gaps of any desired width which are still narrower than any. gap'heretofore produced by sawing' of the toroidal i A-gap slightly wider than tliat produced by fracture .of

' the toroid as herein described may be produced by first spreadmg the gap produced by fracture suificiently to admit a strip of foil of the order of .0O01 of an inch in thickness coated with a lapping material including 'diamond dust or the likejand then lapping the. walls of the fracture'by reciprocating the coated foil in contact'with them. 'This'operation also freesthe walls'from'anyirregularities incident to fracturing of the material. After lapping the walls of the fracture to the desired extenhthe coated foil is removed and a foil shim of the thickness of p the desired gap Visiinserted; 'after'which the tension em ployed to spread the gap is relieved, so that the resiliency stressing force. 7 i i a V ,2. A method of producinga gap in a transducer core A still wider gap may be producedby, anoperation:

identical to that just described except inthat' the force tending to spread the gap is completely relaxed during the sawing operation; A gap produced in thismanner will be of theorder of the thickness of the saw blade and will be less than could have been produced by sawing an unfractured toroid with a saw of'the same thickness. Gaps produced by sawing in the manner herein described will also be completed in'a matter of minutesas compared with a matter of hours of required sawing time, for unfractured toroids of the same material. 7

The outer periphery of the core is next ground'away sufficiently across the gap to slightly flatten the surface; 5 and remove any irregularities due to the. scribing opera-' tion. This flattening is shown to an exaggerated scale, 'in' Fig. 5 at 8, and assists in securing the close spacing to the magnetic medium required. The core is now ready to have coil windings applied, and to' be mounted in a head unit for use.

supra. V

Fig. 6 shows a core 14inplace onfa tapered pin 29.

The taper of pin 29 has been exaggerated in the drawing I for purposes'of illustration. lt'may conveniently have a. taper of .001" per inch of length 'in practice. The pin;29

is clamped in asuitable vise or holder and means'such'as a length ofcylindric-al tubing may be used to applyforcej uniformly to-the toroid114. The vise'and the tubingare conventionahiand neither is shown in the drawing, The

direction of the applied force'should be parallel to the stressing the toroid unevenly.

axis of 'the' pin 29, to avoid -I claim:

1. A method'of producing'a gapin a transducer core. having a; toroidal configuration such as a magne tic'cordf comprising the steps'of grooving thecore at a line'along whichthe gap is to be' pr'oduced and applying a stressing force on said core to separate'the 'core at's aidline until. the core fractures at said line; arresting the application of f such stressing force at the instant of said fracture at'said linewhereb y the core'is'preventied from -breaking other" sorting a dimensioned s than: along said 1ine,- spr'eadingthe gap slightly and in? in said gapg aiidreleasing said V having :a toroidal configuration such as a magnetic core, I

comprising the steps of grooving the core at a line along which the gap is to be produced and applying a stressing force on said core to separate the core until it fractures at said line, arresting the application of such stressing force at the instant of fracture whereby the core is prevented from breaking other than along said line, spreading the gap slightly, inserting an abrasive-charged foil strip into said gap, lapping the opposite faces of said core at said fracture, removing said abrasive-charged strip, inserting a dimensioned shim in said gap and releasing said stressing force.

3. A method of producing a gap in a transducer core having a toroidal configuration such as a magnetic core, comprising the steps of grooving the core at a line along which the gap is to be produced and applying a stressing force on said core to separate the core until the core frac tures at said line, arresting the application of such stressing force at the instant of fracture whereby the core is prevented from breaking other than along said line, spreading the gap slightly, inserting an abrasive-charged foil strip into the gap, releasing the stressing force in said core, reciprocating said strip to lap the opposite fractured surfaces in said core, again spreading said gap, removing said abrasive-charged foil from said gap, inserting a dimensioned shim in said gap, and releasing said stressing force.

4. A method of producing a gap in a transducer core having a toroidal configuration such as a magnetic core, comprising the steps of grooving the core at a line along which the gap is to be produced and applying a stressing force on said core to separate the core until the core fractures at said line, arresting the application of such stressing force at the instant of said fracture whereby the core is prevented from breaking other than along said line, spreading the gap slightly to receive an abrasive-charged saw blade, partially releasing the stressing force on said core, reciprocating said blade to cut away irregularities in the opposed fractured surfaces of said core, removing said blade from said gap, inserting a dimensioned shim in said gap, and releasing said stressing force.

5. A method of producing a gap in a transducer core having a toroidal configuration such as a magnetic core, wherein said gap is smaller in length than the thickness of the tool used for cutting, comprising the steps of grooving the core at a line along which the gap is to be produced, applying a stressing force on said core to separate the core at said line until the core fractures, arresting the application of such stressing force on said core at the instant of fracture whereby the core is prevented from breaking other than along said line, spreading the gap to receive an abrasive-charged saw blade, reciprocating said blade to smooth irregularities in the opposed fractured surfaces of said core, removing said blade from said gap, inserting a shim of thickness less than that of said blade in said gap, and releasing said stressing force.

References Cited in the file of this patent UNITED STATES PATENTS 1,748,993 Purdy Mar. 4, 1930 1,928,458 Mitchell Sept. 26, 1933 2,301,473 Stolberg Nov. 10, 1942 2,320,632 McMaster June 1, 1943 2,325,832 Christensen Aug. 3, 1943 2,346,555 Cobb Apr. 11, 1944 2,608,621 Peterson Aug. 26, 1952 2,621,259 Grantham Dec. 9, 1952 2,627,640 Garnich Feb. 10, 1953 

